This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-184647, filed Jun. 30, 1999; and No. 2000-179544, filed Jun. 15, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a high electron mobility transistor and power amplifier and, more particularly, to a group III nitride inverted high electron mobility transistor and a power amplifier using the same.
Since field effect transistors (FETS) using a group III nitride compound semiconductor material, especially, gallium nitride (GaN) can realize high output power in the high-frequency range, they are expected to be used as power elements. As such transistors, a metal-semiconductor field effect transistor (MESFET), high electron mobility transistor (HEMT), metal-insulator field effect transistor (MISFET), and the like have been proposed.
Among these transistors, according to a GaN-based HEMT which has an AlGaN layer as a donor layer and a GaN layer as a channel layer, higher density of two-dimensional electron gas can be realized in the channel layer compared to a GaAs-based HEMT which has an AlGaAs layer as a donor layer and a GaAs layer as a channel layer. Therefore, the GaN-based HEMT is considered as a very promising high-output element. However, in order to realize high-output characteristics by the GaN-based HEMT, some problems remain unsolved, as will be described below.
FIG. 1 is a schematic sectional view showing a conventional GaN-based normal HEMT. A GaN-based HEMT 110a shown in FIG. 1 has the following structure. That is, a GaN underlayer 114, a GaN channel layer 111, and an Alxcex1Ga(1xe2x88x9260)N (0 less than xcex1 less than 1) donor layer 113 are stacked in turn on a substrate 115, and a gate electrode 116, and source and drain electrodes 117 and 118 are formed on the AlGaN layer 113.
In such structure, a good ohmic contact must be achieved between the source and drain electrodes 117 and 118, and the AlGaN layer 113. However, since the AlGaN layer 113 has a broad band gap, it is hard to achieved a good ohmic contact between the source and drain electrodes 117 and 118, and the AlGaN layer 113. For this reason, in the GaN-based HEMT 110a shown in FIG. 1, the contact resistance between the source and drain electrodes 117 and 118, and the AlGaN layer 113 is large. Hence, the GaN-based normal HEMT structure shown in FIG. 1 cannot realize sufficiently high output.
A structure shown in FIG. 2 is known as the one for combating the problems that have been explained in association with the GaN-based normal HEMT structure shown in FIG. 1. FIG. 2 is a schematic sectional view showing a conventional GaN-based inverted HEMT (IHEMT). In this GaN-based inverted HEMT 110b, an Alxcex1Ga(1xe2x88x92xcex1)N (0 less than xcex1 less than 1) donor layer 113 is formed on a GaN underlayer 114 as in a GaAs-based inverted HEMT. A GaN channel layer 111 is stacked on the AlGaN layer 113, and a gate electrode 116, and source and drain electrodes 117 and 118 are formed on the GaN layer 111 (O. Aktas, et al., IEEE Electron Device letters, Vol. 18, No. 6, pp. 293-295, 1997).
The GaN-based inverted HEMT 110b shown in FIG. 2 is mainly different from the GaN-based normal HEMT 110a shown in FIG. 1 in that the Alxcex1Ga(1xe2x88x92xcex1)N donor layer 113 is located between the GaN channel layer 111 and GaN underlayer 114. In such inverted HEMT, the source and drain electrodes 117 and 118 are formed on the GaN layer 111 unlike the normal HEMT. Also, the band gap of the GaN layer is narrower than that of the AlGaN layer. Therefore, according to the inverted HEMT 110b shown in FIG. 2, the aforementioned problem of the contact resistance can be avoided.
However, the difference between the lattice constants of GaN and Alxcex1Ga(1xe2x88x92xcex1)N is one or more orders of magnitudes larger than that between GaAs and Alxcex1Ga(1xe2x88x92xcex1)As. For this reason, in the GaN-based inverted HEMT 110b shown in FIG. 2, larger piezoelectric charges are produced compared to the GaAs-based inverted HEMT. The piezoelectric charges act to reduce the density of two-dimensional electron gas in the GaN channel layer 111 with respect to the HEMT 110b having the structure shown in FIG. 2. Therefore, even the GaN-based inverted HEMT structure shown in FIG. 2 cannot realize sufficiently high output power.
It is an object of the present invention to provide a group III nitride high electron mobility transistor which can realize high output power.
It is another object of the present invention to provide a power amplifier using a group III nitride high electron mobility transistor which can realize high output power.
According to the first aspect of the present invention, there is provided a group III nitride high electron mobility transistor comprising an underlayer comprising a first group III nitride compound semiconductor material, a donor layer formed on the underlayer and comprising a second group III nitride compound semiconductor material, a lattice constant of a bulk of the donor layer being larger than a lattice constant of the underlayer, a channel layer formed on the donor layer and comprising a third group III nitride compound semiconductor material, and gate, source, and drain electrodes formed on the channel layer.
According to the second aspect of the present invention, there is provided a group III nitride high electron mobility transistor comprising an underlayer comprising AlxGa(1xe2x88x92x)N, a donor layer formed on the underlayer and comprising AlyGa(1xe2x88x92y)N, x and y satisfying an inequality 0xe2x89xa6y less than xxe2x89xa61, a channel layer formed on the donor layer and comprising a nitrogen compound, and gate, source, and drain electrodes formed on the channel layer.
According to the third aspect of the present invention, there is provided a power amplifier comprising a group III nitride high electron mobility transistor. which comprises an underlayer comprising a first group III nitride compound semiconductor material, a donor layer formed on the underlayer and comprising a second group III nitride compound semiconductor material, a lattice constant of a bulk of the donor layer being larger than a lattice constant of the underlayer, a channel layer formed on the donor layer and comprising a third group III nitride compound semiconductor material, and gate, source, and drain electrodes formed on the channel layer, an input terminal receiving an input signal and is connected to the gate electrode, an output terminal outputting an output signal and connected to the drain electrode, and a power source connected to the drain electrode via a choke coil.
According to the fourth aspect of the present invention, there is provided a power amplifier comprising group III nitride high electron mobility transistor comprising an underlayer comprising AlxGa(1xe2x88x92x)N, a donor layer formed on the underlayer and comprising AlyGa(1xe2x88x92y)N, x and y satisfying an inequality 0xe2x89xa6y less than xxe2x89xa61, a channel layer formed on the donor layer and comprising a nitrogen compound, and gate, source, and drain electrodes formed on the channel layer, an input terminal receiving an input signal and connected to the gate electrode, an output terminal outputting an output signal and connected to the drain electrode, and a power source connected to the drain electrode via a choke coil.
As described above, the conventional GaN-based IHEMT has a structure in which an Alxcex1Ga(1xe2x88x92xcex1)N (0 less than xcex1 less than 1) donor layer having a smaller lattice constant than that of GaN is stacked on a GaN underlayer. In such structure, since positive piezoelectric charges are produced near the interface of the donor layer with the underlayer, and negative piezoelectric charges are produced near the interface of the donor layer with the channel layer, the density of two-dimensional electron gas in the channel layer lowers. For this reason, the conventional GaN-based IHEMT cannot realize sufficiently high output power.
By contrast, in a group III nitride IHEMT of the present invention, the lattice constant of the bulk of the donor layer is larger than that of the underlayer. Therefore, when a combination of materials that can produce negative piezoelectric charges near the interface of the donor layer with the underlayer is selected from those that satisfy the aforementioned condition, the density of two-dimensional electron gas in the channel layer can be sufficiently raised. That is, sufficiently high output power can be realized. Note that the term xe2x80x9cbulkxe2x80x9d used herein means a solid which extends three-dimensionally, and has a size large enough to ignore effects of surfaces, interfaces, and edges.
In order to produce the aforementioned piezoelectric charges, for example, AlxGa(1xe2x88x92x)N can be used as the first group III nitride compound semiconductor material that forms the underlayer, and AlyGa(1xe2x88x92y)N can be used as the second group III nitride compound semiconductor material that forms the donor layer (x and y satisfy inequality 0xe2x89xa6y less than xxe2x89xa61). When these materials are used as the first and second group III nitride compound semiconductor materials, very high output power can be realized.
In the group III nitride IHEMT of the present invention, for example, GaN can be used as the third group III nitride compound semiconductor material that forms the channel layer. Alternatively, InzGa(1xe2x88x92z)N may be used as the third group III nitride compound semiconductor material (z satisfies inequality 0 less than z less than 1).
The group III nitride IHEMT of the present invention may have a spacer layer, which comprises the fourth group III nitride compound semiconductor material, between the donor layer and channel layer. For example, when the aforementioned materials are used as the first and second group III nitride compound semiconductor materials, AlyGa(1xe2x88x92y)N can be used as the fourth group III nitride compound semiconductor material.
In the group III nitride IHEMT of the present invention, the underlayer may be a thin film formed on a substrate such as a wafer, or may be a substrate itself such as a wafer. In the former case, the underlayer can be obtained by crystal growth of the first nitrogen compound on the substrate. In this case, it is preferable that a nucleation layer that provides a nucleus for that crystal growth be formed on the substrate, and crystals of first group III nitride compound semiconductor material then be grown.
In the group III nitride IHEMT of the present invention, the density of negative piezoelectric charges produced near the interface of the donor layer with the underlayer is preferably about 1010/cmxe2x88x922 or higher when it is divided by the absolute value of electron charges. On the other hand, the difference between the lattice constants of the bulk of the donor layer and the underlayer is preferably 0.0009% or higher with respect to the lattice constant of the bulk of the donor layer. Alternatively, the difference between the molar ratios of Al in the donor layer and that in the underlayer is preferably 0.00035 or more. In these cases, the effect of increasing the density of two-dimensional electron gas in the channel layer improves.
The lattice constants and compositions of the individual layers that form the group III nitride IHEMT of the present invention can be measured by known methods such as X-ray diffraction, elemental analysis, and the like. That is, when the donor layer consists of mixed crystals like AlyGa(1xe2x88x92y)N, if the molar ratios of the constituent elements are obtained by a known method, the lattice constant of that bulk can be calculated using the vegard""s law.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.